More than 30 years ago the outgoing leader of the SNP declared Scotland to be “England’s last colony”.

Gordon Wilson, who was about to be replaced by his 35-year-old deputy, Alex Salmond, had riffed on this idea before. 

The former MP had helped author his party’s “It’s Scotland’s oil” slogan. The North Sea bonanza, he argued, was being squandered by politicians in London, or the  “imperial capital” as the city was called in at least one press release.

So when Mr Wilson in 1990 talked of Scotland as a colony he was rehearsing something very close to SNP orthodoxy.

Back in the 1970s and 1980s many mainstream nationalists still saw their country as a victim of imperialism. Now, three decades later, most regard Scotland as a perpetrator.

Scotland, at least according to the changing dominant ideology of the SNP, has morphed from “colonised" to “coloniser”. This is quite a turnaround, an historic intellectual redefining of the independence movement’s core beliefs.

As so often with slow but profound change the transformation of SNP thinking on imperialism has largely gone unnoticed - or at least unappreciated - by journalists and commentators 

But it has not been passed by historians. Scholars Stephen Mullen and Ewan Gibbs, both of Glasgow University, have documented the shift in thinking - and how it dovetailed with revelatory recent research on the sheer scale of Scotland’s role in chattel slavery and other imperial crimes against humanity.

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Their paper, Scotland, Atlantic Slavery, and the Scottish National Party: from Colonised to Coloniser in the Political Imagination, will be published in an academic journal, Nations and Nationalism, in the New Year. 

“Our analysis cuts through the polemic, tracing a shift in dominant SNP thinking that once believed Scotland to be a colony to the current acknowledgement of Scots as enslavers,” Mr Mullen told The Herald. “It underlines the major impact that historians have had on public discourse and relevant government policy in Scotland.”

Senior nationalist figures have started taking responsibility for exploitation of trafficked Africans and their descendants by Scots planters in Caribbean colonies. 

Much of the money made by slave owners made its way back to this country. Some of it, as Mr Mullen revealed in his recent book Glasgow’s Sugar Aristocracy, is still benefiting Scotland.

Earlier this year the SNP leader of Glasgow City Council, Susan Aitken, formally apologised for her city’s role in chattel slavery. 

Other politicians are starting to make similar noises. 

Saying sorry has been dismissed as “mere tokenism” by Sir Tom Devine, the retired professor at Edinburgh University whose work in the late 20th century first started to explain the importance of “Atlantic trade” to modern Scotland.

The detailed research which followed Sir Tom's - not least by Mr Mullen - has demonstrated just how much Scotland’s sugar “merchants” depended on slavery and how their wealth percolated through the country. 

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Ms Aitken’s apology came, for example, after the council commissioned Mr Mullen to look in to its own links to slavery. The historian found half of the city’s lords provost between 1636 and 1834 had connections to slave-linked commerce.

Civic leaders from other parties have followed suit. The Liberal Democrat Lord Provost of Edinburgh, Robert Aldridge, formally said sorry for his city’s role in sustaining slavery and colonialism back in October. But his party, of course, has never viewed Scotland as a colony.

Revelations about slave profits have prompted more than public expressions of sorrow and regret. They have gone beyond the token. There are specific reparations, such as those offered by Glasgow University, an ongoing beneficiary of laundered slave money. An understanding of the past is informing policy in the present.

Nationalist politics, the Gibbs and Mullen paper shows, has developed along with the historiography. 

Until quite recently even distinguished scholars failed to explore the role of Scots and Scotland in slavery and imperialism.

“From 1956 until the late 1970s, an influential group of historians, including T.C. Smout, downplayed the extent to which Atlantic commerce (and extension, chattel slavery) contributed to Scottish development,” wrote Mr Gibbs and Mr Mullen in their paper. “Historians advancing ‘enclave’ arguments argued trade in colonial produce (especially tobacco) was monopolised by a few Glasgow merchants and was an ‘enclave’ within a Scottish economy developing independently of colonial processes. Thus, the influential ‘enclave’ school claimed Atlantic commerce and merchant capital had few multiplier effects on the wider Scottish economy.

They added: “T.M. Devine’s work from 1971 onwards, by contrast, illustrated the broader significance of merchant capital and the Atlantic trades to Scottish agricultural and industrial development. “It was to be, however, another forty years before chattel slavery was accepted as the foundation which supported these revolutionary processes.

It is in the last decade that many of the breakthroughs have taken place in the scholarship. That does not mean there is not public and more rarely academic resistance to new revelations. And there remains confusion over the legacy of chattel slavery.

Some people conflate the “slave trade” and “slavery”. The former was abolished in the British Empire in 1807, the latter in 1833.

Scottish merchants were not heavily involved in moving people from Africa to the Americas. Their role was different, in exploiting, not trafficking, enslaved people. And we now know how far cotton, sugar and other slave-derived goods and profits went to fuel industrialisation and modernisation in Scotland.

“Since the late 1990s, the scholarship around Scotland’s connections with Atlantic slavery has been transformed,” explained Mr Gibbs and Mr Mullen. “It is now largely accepted that relatively few ‘triangular trade’ voyages (just 27 are recorded) departed Scottish ports between 1706 and 1766. However, Scots were disproportionately involved in trading slave-grown produce.”

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There was far less public understanding of Scotland’s role in slavery when Mr Wilson declared the country to be a colony back in 1990.

His leadership had been marked by failure and intellectual torpor.  The SNP got just 14% of the vote in the 1987 general election, a slight rise from 1983 but less than the supposedly hated Tories of Margaret Thatcher. 

Mr Wilson had lost his own seat. But Mr Gibbs and Mr Mullen stress that there had been more progressive views of empire and imperialism in the party. Indeed, there were contradictions and disagreement about empire and imperialism inside the SNP from its very inception. Mr Wilson took office in 1979 after defeating an intellectual called Stephen Maxwell for the post. 

Mr Maxwell - inspired by Scottish nationalist thinker Tom Nairn - had a very different take on whether Scotland was a colony. As early as 1982, Mr Maxwell was stressing that Scotland was not just a junior partner in empire, but a "major beneficiary of the slave trade”.

“Maxwell’s observations anticipated the conclusions of Scottish historians and nationalist politicians by decades,” wrote Mr Gibbs and Mr Mullen.

"Whereas the mid-twentieth century setting had been one of imagining an independent Scotland joining the postcolonial world order, towards the end of the century both cultural and political forces (in local and international contexts), as well as historiography, shaped a more nuanced national past which incorporated historic culpability in Atlantic slavery.”

Mr Wilson’s theory of Scotland as a colony may no longer be mainstream. But that does not mean that nationalists never indulge whataboutery on our past. 

One now retired cabinet minister, Mike Russell, earlier this month compared the UK’s refusal to allow a second independence referendum with “the old colonial approach to self-determination”.

Mr Russell stressed that Scotland did not meet the “classic definition” of a colony. This qualifier did not satisfy his critics. Some independence supporters still use the rhetoric of Mr Wilson. 

Others - as documented by Mr Gibbs and Mr Mullen - feel the need to cite Scottish victimhood even when acknowledging the country’s crimes. 

And this, as the historians pointed out, can have real world policy implications. For example, Stuart McMillan, the SNP MSP, Greenock and Inverclyde, responded to calls for a national museum to highlight Scotland’s role in the slave trade and colonialism’ by calling for crimes against Scots, such as the Highland Clearances, to be incorporated.

This chimes with a long tradition of Scottish nationalists appropriating the oppression of Highlanders (or working-class Lowlanders) for the whole nation.  

So the SNP may no longer see Scotland as a colony, and it may have adopted contemporary anti-imperialist and even BLM narratives in a way UK Tories have not. But its politicians still play the victim card when they feel they need to.

“By embracing Scotland’s historic role in Atlantic slavery, the SNP has juxtaposed itself with the imperial revanchism which influences the Conservative Party,” summed up Mr Gibbs. “But Scottish nationalists also retain a tendency to ‘balance’ Scottish culpability with oppression suffered by Scots or positive stories related to abolition of slavery.”